Written by Wydun, because he loves you.
Ninja Guide:

Lotus Ninja: Is the heavy minded melee ninja. The first strike kind of ninja, relies heavily on getting a backstab off to open the fight. 

Weapons: You have two options you can Dual wield with a Piercing weapon (dagger). If you go that route i recommend using a Relic dagger, I prefer the Fleshripper just because I like the stats. And the 
offhand weapon can be any other relic weapon you want. As long as you make sure your dagger is in your righthand you'll be golden. 

The other option for weapon is a 2handed Trident called Neptune's Trident. With this weapon I believe it is possible to get the HIGHEST damage out put on a single backstab. It will come down to 
personal preference on which you choose or not.

Stating: Lotus ninja is a Strength first kind of class. Strength increases your damage for backstab/melee/shishou (mid rounder). Secondly is Agility, this will allow for all your throws to land and 
your melee to hit. Ideally you could have agility as your primary but its not a must do. Your third choice for a stat is between Dexterity or Constitution in my book. I usually choose Dexterity because 
of the relic choices. But constitution does work out.

Backstab: Strength + Damroll + Weapon Average + Kakusu (your invis) = Damage out put.
Bonus to backstab damage is easily done by using Throw Bomb and putting your victim to sleep and landing that NASTY NASTY backstab that lotus can do.
Mid rounder: I prefer shishou its an ability you must time to make sure you land it 100% but its the most consistant damage for a lotus. Other options are Suro and Throw Darts.

Serpent Ninja: 
Is the hybrid type of ninja in my book. They require a venom toxin to enter their enemies body in order to increase their own abilities.
Weapons: ANY absolutely any weapon you want. But remember if you want to use shishou you must use a Stab or Pierce weapon...

You have a couple different ways to stat.
1. Intelligence / Agility / Strength = The pure offensive build, with your venom debuff you should be able to out damage most classes if played carefully.
2. Intelligence / Dexterity / Agility = The tankier build with using throw darts as your middie
3. Intelligence / Dexterity / Constitution = The anti melee build, this build requires you to hope they have low dex so your darts can land

Evenom: Set up for 5 stat and 3 hp is the most common. Tweak with it as you want. (skill can be used anytime during the fight)
Throw Darts: Your main damage source
Backstab: Is possible to use but in my personal play style it was more of a burden then a benefit.
Shishou: Is viable if you have Strength otherwise stick with darts.

Spider Ninja: 
This ninja type is all about laying down the traps and laughing at the victims that walk on them.
Weapons: Any personal preference. Once again Stab/Pierce if you want to backstab/Shishou.

Intelligence is your main stat followed by dexterity.
Knowing you have those two stats as a need. You also should look at getting Strength and Agility to help your melee when you're caught in a fight.
Traps are fun....
Explosion Trap: Damage damage damage
Confusion: Confuses the individual so when in combat he/she may do damage to themselves
Needles: its a debuff!!!!
Poison: It's a Damage over Time
Sleep: Puts them to sleep which will cause greater damage for backstab or an explosion trap 
Web: Places a web binding around the individual. Just like bola it restrains them until the flex out of it.
Also remember you can have 5 traps set at one, but only 3 will work per room. So ideally set a couple in a few rooms and grapple them to you and laugh at the damage they have taken and the affects they 
suffer with.